Natural killer (NK) cells are a third subtype of lymphocytes besides B and T cells. NK cells provide an important immune function in the defense against viruses and other intracellular pathogens. Unlike B and T cells, NK cells do not exhibit specificity for antigen and they acquired their name because they can kill cells without prior stimulation by antigen. The killing of normal healthy cells is prevented by inhibitory receptors on NK cells that recognize major histocompatibility class I molecules. A large number of receptors that activate the cytotoxic response of NK cells have been described. However, it is not clear if any one of these receptors is sufficient and/or necessary to activate natural cytotoxicity. The major goal of this project is to define the molecular basis of NK cell activation and to characterize the receptors involved. A reconstitution system has been developed to test for the contribution of individual receptors to the activation of NK cell cytotoxicity. Several specific ligands of NK cell receptors have been expressed in an insect cell line in order to determine the minimal requirements for the activation of NK cells. Activated natural killer (NK) cells lyse tumor cells and virus-infected cells and produce interferon-gamma upon contact with sensitive target cells. The regulation of these effector responses in resting NK cells is not well understood. A receptor, KIR2DL4, was described that has the unique property of inducing interferon-gamma production, but not cytotoxicity, by resting NK cells in the absence of cytokines. In contrast, other NK cell activation receptors induced cytotoxicity but not interferon-gamma production. The induction by KIR2DL4 of IFN-gamma production by resting NK cells was blocked by an inhibitor of the p38 mitogen activated protein kinase (MAPK) signaling pathway, in contrast to the interleukin-2-induced interferon-gamma secretion that was sensitive to inhibition of the Erk MAPK pathway. These results reveal a functional dichotomy (cytokine production versus cytotoxicity) in the response of resting NK cells, as dictated by the signals of individual receptors. The gp49B receptor has an inhibitory activity in both NK cells and mast cells. Mice deficient in the gp49B gene have been generated. Such gp49-deficient mice have been crossed for several generations with a congenic strain of mice in order to obtain a fairly pure genetic background. NK cells developed normally in gp49B-deficient mice and displayed normal cytotoxic activity in vitro. Likewise, mast cells developed normally in gp49B-deficient mice and displayed normal degranulation activity in vitro. The healthy status of gp49B-null mice makes them very suitable for testing the role of gp49B in controlling immune responses during infections, in particular those known to elicit NK or mast cell responses.